The process and apparatus of the invention is intended for use in the field of injection-blow molding of thermoplastic articles, and comprises methods and apparatus for forming thermoplastic articles with accurately controlled wall thicknesses. The process and apparatus of the invention finds particular application in the production of relatively thin walled thermoplastic articles where wall thickness dimensions are extremely important. The invention further includes the sub-process and apparatus for blow molding certain nestable thermoplastic preforms into non-nestable relatively thin walled containers with accurately controlled wall thicknesses.
In the injection molding art wherein the male mold is subject to deflection by an unbalanced flow of the molten thermoplastic material injected into the mold cavity under relatively high pressures, defective articles are commonly produced. Injection molding of thermoplastic articles is substantially improved through the use of the process and apparatus of my co-pending applications, Ser. No. 308,200, filed Nov. 20, 1972 now Pat. No. 3,820,931 and Ser. No. 371,652, filed June 19, 1973 now Pat. No. 3,829,548, the subject matter of which is incorporated by reference herein.
In the blow molding of thermoplastic preforms through the use of blow molding equipment, it has often been found that improper wall thicknesses result when the preform is blown to the desired reform configuration. Such containers are often designed with a wall thickness of about 0.008 to 0.013 inches and in known injection-blow molding equipment or in known blow molding equipment using preforms, the preform will often shift on the male mold or mandrel during blowing and this will cause the formation of defective containers with holes through a side wall or with an excessively thin wall at one portion with an excessively thick wall at another portion of the container.
The processes and apparatus of the subject invention lend themselves ideally to the manufacture of disposable thin walled thermoplastic cups or containers. In blow molding such containers use is made of thermoplastic preforms such as shown in my co-pending application, Ser. No. 214,275, filed Dec. 30, 1971, now abandoned, the subject matter of which is incorporated by reference herein.